The present invention relates to a liquid-crystalline medium, and to the use thereof for electro-optical purposes and displays containing this medium.
Liquid crystals are used, in particular, as dielectrics in display devices, since the optical properties of such substances can be modified by an applied voltage. Electro-optical devices based on liquid crystals are extremely well known to the person skilled in the art and can be based on various effects. Examples of such devices are cells having dynamic scattering, DAP (deformation of aligned phases) cells, guest/host cells, TN (twisted nematic) cells, STN (supertwisted nematic) cells, SBE (superbirefringence effect) cells and OMI (optical mode interference) cells. The most common display devices are based on the Schadt-Helfrich effect and have a twisted nematic structure.
The liquid-crystal materials must have good chemical and thermal stability and good stability to electric fields and electromagnetic radiation. Furthermore, the liquid-crystal materials should have relatively low viscosity and give short addressing times, low threshold voltages and high contrast in the cells.
Furthermore, they should have a suitable mesophase, for example a nematic or cholesteric mesophase for the abovementioned cells, at conventional operating temperatures, i.e. in the broadest possible range above and below room temperature. Since liquid crystals are generally used in the form of mixtures of a plurality of components, it is important that the components are readily miscible with one another. Further properties, such as the electrical conductivity, the dielectric anisotropy and the optical anisotropy, must satisfy different requirements depending on the cell type and area of application. For example, materials for cells having a twisted nematic structure should have positive dielectric anisotropy and low electrical conductivity.
For example, media of large positive dielectric anisotropy, broad nematic phases, relatively low birefringence, very high resistivity, good UV and temperature stability and low vapour pressure are desired for matrix liquid-crystal displays having integrated nonlinear elements for switching individual pixels (MLC displays).
Matrix liquid-crystal displays of this type are known. Examples of nonlinear elements which can be used for individual switching of individual pixels are active elements (i.e. transistors). This is then referred to as an xe2x80x9cactive matrixxe2x80x9d, and a differentiation can be made between two types:
1. MOS (metal oxide semiconductor) or other diodes on silicon wafers as substrate.
2. Thin-film transistors (TFTs) on a glass plate as substrate.
Use of monocrystalline silicon as the substrate material limits the display size, since even modular assembly of the various part-displays results in problems at the joints.
In the case of the more promising type 2, which is preferred, the electro-optical effect used is usually the TN effect. A differentiation is made between two technologies: TFTs comprising compound semiconductors, such as, for example, CdSe, or TFTs based on polycrystalline or amorphous silicon.
The TFT matrix is applied to the inside of one glass plate of the display, whilst the other glass plate carries the transparent counterelectrode on the inside. Compared with the size of the pixel electrode, the TFT is very small and has virtually no adverse effect on the image. This technology can also be extended to fully colour-compatible image displays, where a mosaic of red, green and blue filters is arranged in such a way that each filter element is located opposite a switchable pixel.
The TFT displays usually operate as TN cells with crossed polarizers in transmission and are illuminated from the back.
The term MLC displays here covers any matrix display containing integrated nonlinear elements, i.e., in addition to the active matrix, also displays containing passive elements, such as varistors or diodes (MIM=metal-insulator-metal).
MLC displays of this type are particularly suitable for TV applications (for example pocket TV sets) or for high-information displays for computer applications (laptops) and in automobile or aircraft construction. In addition to problems with respect to the angle dependence of the contrast and the response times, problems arise in MLC displays owing to inadequate resistivity of the liquid-crystal mixtures [TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORIMACHI, K., TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, September 1984: A 210-288 Matrix LCD Controlled by Double Stage Diode Rings, p. 141 ff, Paris; STROMER, M., Proc. Eurodisplay 84, September 1984: Design of Thin Film Transistors for Matrix Addressing of Television Liquid Crystal Displays, p. 145 ff, Paris]. With decreasing resistance, the contrast of an MLC display drops, and the problem of after-image elimination can occur. Since the resistivity of the liquid-crystal mixture generally drops over the life of an MLC display owing to interaction with the internal surfaces of the display, a high (initial) resistance is very important in order to obtain acceptable service lives. In particular in the case of low-voltage mixtures, it was hitherto not possible to achieve very high resistivities. It is furthermore important that the resistivity increases as little as possible with increasing temperature and after heating and/or exposure to UV radiation. Also particularly disadvantageous are the low-temperature properties of the mixtures from the prior art. It is required that crystallization and/or smectic phases do not occur, even at low temperatures, and that the temperature dependence of the viscosity is as low as possible. MLC displays of the prior art thus do not satisfy current requirements.
There thus continues to be a great demand for MLC displays having very high resistivity at the same time as a broad operating temperature range, short response times, even at low temperatures, and low threshold voltage which do not have these disadvantages or only do so to a reduced extent.
In the case of TN (Schadt-Helfrich) cells, media are desired which facilitate the following advantages in the cells:
broadened nematic phase range (in particular down to low temperatures),
switchability at extremely low temperatures (outdoor use, automobiles, avionics),
increased stability on exposure to UV radiation (longer life).
The media available from the prior art do not enable these advantages to be achieved while simultaneously retaining the other parameters.
In the case of supertwisted cells (STN), media are desired which enable greater multiplexibility and/or lower threshold voltages and/or broader nematic phase ranges (in particular at low temperatures). To this end, a further extension of the parameter latitude available (clearing point, smectic-nematic transition or melting point, viscosity, dielectric quantities, elastic quantities) is urgently desired.
The invention has an object of providing media, in particular for MLC, TN or STN displays of this type, and also in-plane switching (IPS) displays, which do not have the abovementioned disadvantages, or only do so to a reduced extent, and preferably at the same time have very high resistivities and low threshold voltages. In particular it is possible, using the compounds of the formula I, to prepare low Vth mixtures having a very good Vth/xcex31 ratio.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
It has now been found that objects such as these can be achieved when novel media are used in displays.
The invention thus includes a liquid-crystalline medium based on a mixture of polar compounds having positive dielectric anisotropy, characterized in that it comprises one or more compounds of the general formula I, 
in which
R is an alkyl radical having 1 to 12 carbon atoms which is unsubstituted, monosubstituted by CN or CF3 or at least monosubstituted by halogen, it also being possible for one or more CH2 groups in these radicals to be replaced, in each case independently of one another, by xe2x80x940xe2x80x94, xe2x80x94Sxe2x80x94, 
xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94or xe2x80x94Oxe2x80x94COxe2x80x94Oxe2x80x94 in such a way that O atoms are not linked directly to one another, 
L1-4 are, in each case independently of one another, H or F,
X is F, Cl, CN, OCN, NCS, SCN, halogenated alkyl radical, halogenated alkenyl radical, halogenated alkoxy radical or halogenated alkenyloxy radical having up to 6 carbon atoms, and
u is0 or 1.
The compounds of the formula I have a broad range of applications. Depending on the choice of substituents, these compounds can serve as base materials from which liquid-crystalline media are predominantly composed; however, compounds of the formula I can also be added to liquid-crystalline base materials from other classes of compound in order, for example, to modify the dielectric and/or optical anisotropy of a dielectric of this type and/or to optimize its threshold voltage and/or its viscosity.
In the pure state, the compounds of the formula I are colourless and form liquid-crystalline mesophases in a temperature range which is favourably located for electro-optical use. They are stable chemically, thermally and to light.
If R1 is an alkyl radical and/or an alkoxy radical, this can be straight-chain or branched. It is preferably straight-chain, has 2, 3, 4, 5, 6 or 7 carbon atoms and accordingly is preferably ethyl, propyl, butyl, pentyl, hexyl, heptyl, ethoxy, propoxy, butoxy, pentoxy, hexoxy or heptoxy, furthermore methyl, octyl, nonyl, decyl, undecyl, dodecyl, methoxy, octoxy, nonoxy, decoxy or undecoxy.
Oxaalkyl is preferably straight-chain 2-oxapropyl (xe2x95x90methoxymethyl), 2-(xe2x95x90ethoxymethyl) or 3-oxabutyl (=2-methoxyethyl), 2-, 3- or 4-oxapentyl, 2-, 3-, 4- or 5-oxahexyl, 2-, 3-, 4-, 5- or 6-oxaheptyl, 2-, 3-, 4-, 5-, 6- or 7-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 8-oxanonyl, or 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-oxadecyl.
If R1 is an alkyl radical in which one CH2 group has been replaced by xe2x80x94CHxe2x95x90CHxe2x80x94, this can be straight-chain or branched. It is preferably straight-chain and has 2 to 10 carbon atoms. Accordingly, it is in particular vinyl, prop-1- or -2-enyl, but-1-, -2- or -3-enyl, pent-1-, -2-, -3- or -4-enyl, hex-1-, -2-, -3-, -4- or -5-enyl, hept-1-, -2-, -3-, -4-, -5- or -6-enyl, oct-1-, -2-, -3-, -4-, -5-, -6- or -7-enyl, non-1-, -2-, -3-, -4-, -5-, -6-, -7- or -8-enyl, or dec-1-, -2-, -3-, -4-, -5-, -6-, -7-, -8- or -9-enyl.
If R is an alkyl radical in which one CH2 group has been replaced by xe2x80x94Oxe2x80x94 and one has been replaced by xe2x80x94COxe2x80x94, these are preferably adjacent. These thus contain one acyloxy group xe2x80x94COxe2x80x94Oxe2x80x94 or one oxycarbonyl group xe2x80x94Oxe2x80x94COxe2x80x94. These are preferably straight-chain and have 2 to 6 carbon atoms. Accordingly, they are in particular acetoxy, propionyloxy, butyryloxy, pentanoyloxy, hexanoyloxy, acetoxymethyl, propionyloxymethyl, butyryloxymethyl, pentanoyloxymethyl, 2-acetoxyethyl, 2-propionyloxy-ethyl, 2-butyryloxyethyl, 3-acetoxypropyl, 3-pro-pionyloxypropyl, 4-acetoxybutyl, methoxycarbonyl, ethoxycarbonyl, propoxycarbonyl, butoxycarbonyl, pentoxycarbonyl, methoxycarbonylmethyl, ethoxycarbonyl-methyl, propoxycarbonylmethyl, butoxycarbonylmethyl, 2-(methoxycarbonyl)ethyl, 2-(ethoxycarbonyl)ethyl, 2-(propoxycarbonyl) ethyl, 3-(methoxycarbonyl)propyl, 3-(ethoxycarbonyl)propyl or 4-(methoxycarbonyl)butyl.
If R1 is an alkyl radical in which one CH2 group has been replaced by unsubstituted or substituted xe2x80x94CHxe2x95x90CHxe2x80x94 and an adjacent CH2 group has been replaced by CO or COxe2x80x94O or Oxe2x80x94CO, this can be straight-chain or branched. It is preferably straight-chain and has 4 to 12 carbon atoms. Accordingly, it is in particular acryloyloxymethyl, 2-acryloyloxyethyl, 3-acryloyloxypropyl, 4-acryloyloxybutyl, 5-acryloyloxypentyl, 6-acryloyloxyhexyl, 7-acryloyloxyheptyl, 8-acryloyloxyoctyl, 9-acryloyloxynonyl, 10-acryloyloxydecyl, methacryloyloxymethyl, 2-methacryloyloxyethyl, 3-methacryloyloxypropyl, 4-methacryloyloxybutyl, 5-methacryloyloxypentyl, 6-methacryloyloxyhexyl, 7-methacryloyloxyheptyl, 8-methacryloyloxyoctyl and 9-methacryloyloxynonyl.
If R1 is an alkyl or alkenyl radical which is monosubstituted by CN or CF3, this radical is preferably straight-chain. The substitution by CN or CF3 is in any desired position.
If R1 is an alkyl or alkenyl radical which is at least monosubstituted by halogen, this radical is preferably straight-chain and halogen is preferably F or Cl. In the case of multiple substitution, halogen is preferably F. The resultant radicals also include perfluorinated radicals. In the case of monosubstitution, the fluorine or chlorine substituent can be in any desired position, but preferably in the xcfx89-position.
Compounds of the formula I which contain wing groups R1 which are suitable for polyaddition reactions are suitable for the preparation of liquid-crystalline polyaddition products.
Compounds of the formula I containing branched wing groups R1 may occasionally be of importance due to better solubility in the customary liquid-crystalline base materials, but in particular as chiral dopes if they are optically active. Smectic compounds of this type are suitable as components for ferroelectric materials.
Compounds of the formula I having SA phases are suitable, for example, for thermally addressed displays.
Branched groups of this type generally contain not more than one chain branch. Preferred branched radicals R1 are isopropyl, 2-butyl (=1-methylpropyl), isobutyl (=2-methylpropyl), 2-methylbutyl, isopentyl (=3-methylbutyl), 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl, 2-propylpentyl, isopropoxy, 2-methylpropoxy, 2-methylbutoxy, 3-methylbutoxy, 2-methylpentoxy, 3-methylpentoxy, 2-ethylhexoxy, 1-methylhexoxy and 1-methylheptoxy.
If R1 is an alkyl radical in which two or more CH2 groups have been replaced by xe2x80x94Oxe2x80x94 and/or xe2x80x94COxe2x80x94Oxe2x80x94, this may be straight-chain or branched. It is preferably branched and has 3 to 12 carbon atoms. Accordingly, it is in particular biscarboxymethyl, 2,2-biscarboxyethyl, 3,3-biscarboxypropyl, 4,4-biscarboxybutyl, 5,5-biscarboxypentyl, 6,6-biscarboxyhexyl, 7,7-biscarboxyheptyl, 8,8-biscarboxyoctyl, 9,9-biscarboxynonyl, 10,10-biscarboxydecyl, bis(methoxycarbonyl)methyl, 2,2-bis(methoxycarbonyl)ethyl, 3,3-bis(methoxycarbonyl)-propyl, 4,4-bis(methoxycarbonyl)butyl, 5,5-bis(methoxycarbonyl)pentyl, 6,6-bis(methoxycarbonyl)hexyl, 7,7-bis(methoxycarbonyl)heptyl, 8,8-bis(methoxycarbonyl)-octyl, bis(ethoxycarbonyl)methyl, 2,2-bis(ethoxycarbonyl)ethyl, 3,3-bis(ethoxycarbonyl)propyl, 4,4-bis(ethoxycarbonyl)butyl and 5,5-bis(ethoxycarbonyl)-hexyl.
The compounds of the formula I are prepared by methods known per se, as described in the literature (for example in the standard works, such as Houben-Weyl, Methoden der Organischen Chemie, Georg-Thieme-Verlag, Stuttgart) or in DE 199 06 254 A1, to be precise under reaction conditions which are known and suitable for said reactions. Use can also be made here of variants which are known per se, but which are not mentioned here in greater detail.
The invention also relates to electro-optical displays (in particular STN or MLC displays having two plane-parallel outer plates which, together with a frame, form a cell, integrated nonlinear elements for switching individual pixels on the outer plates, and a nematic liquid-crystal mixture of positive dielectric anisotropy and high resistivity located in the cell) which contain media of this type, and to the use of these media for electro-optical purposes. The mixtures according to the invention are likewise suitable for IPS applications (In Plane Switching).
The liquid-crystal mixtures according to the invention facilitate a significant broadening of the parameter latitude available.
The achievable combinations of clearing point, rotation viscosity xcex31 and dielectric anisotropy are far superior to previous materials from the prior art.
The requirement for a high clearing point, a nematic phase at low temperature and a high xcex94xcex5 was previously only achievable to an unsatisfactory extent. Although systems such as, for example, MLC-6424 have similar properties to the mixtures according to the invention, they have, however, clearly poorer values for the rotational viscosity xcex31.
Other mixture systems have comparable flow viscosities xcexd20 and values of xcex94xcex5, but only have clearing points in the region of 60xc2x0 C.
The liquid-crystal mixtures according to the invention make it possible to achieve clearing points of above 80xc2x0, preferably above 90xc2x0, particularly preferably above 100xc2x0 C., and simultaneously dielectric anisotropy values xcex94xcex5xe2x89xa76, preferably xe2x89xa78, and a high value for the resistivity while retaining the nematic phase down to xe2x88x9220xc2x0 C. and preferably down to xe2x88x9230xc2x0 C., particularly preferably down to xe2x88x9240xc2x0 C., which allows excellent STN and MLC displays to be achieved. In particular, the mixtures are characterized by low operating voltages. The TN thresholds are preferably below 2.0 V, more preferably below 1.5 V, particularly preferably  less than 1.3 V.
It goes without saying that a suitable choice of the components of the mixtures according to the invention also allows higher clearing points (for example above 110xc2x0) to be achieved at higher threshold voltages or lower clearing points to be achieved at lower threshold voltages while retaining the other advantageous properties. It is likewise possible to obtain mixtures of relatively high xcex94xcex5 and thus relatively low thresholds if the viscosities are increased by a correspondingly small amount. The MLC displays according to the invention preferably operate in the first transmission minimum of Gooch and Tarry [C. H. Gooch and H. A. Tarry, Electron. Lett. 10, 2-4, 1974; C. H. Gooch and H. A. Tarry, Appl. Phys., Vol. 8, 1575-1584, 1975]; in this case, a lower dielectric anisotropy in the second minimum is sufficient in addition to particularly favourable electro-optical properties, such as, for example, high gradient of the characteristic line and low angle dependency of the contrast (German Patent 30 22 818) at the same threshold voltage as in an analogous display. This allows significantly higher resistivities to be achieved in the first minimum using the mixtures according to the invention than using mixtures containing cyano compounds. A person skilled in the art can use simple routine methods to produce the birefringence necessary for a prespecified layer thickness of the MLC display by a suitable choice of the individual components and their proportions by weight.
The flow viscosity xcexd20 at 20xc2x0 C. is preferably  less than 60 mm2.sxe2x88x921, particularly preferably  less than 50 mm2.sxe2x88x921. The rotational viscosity xcex31 of the mixtures according to the invention at 20xc2x0 C. is preferably  less than 200 mPa.s, particularly preferably  less than 180 mpa.s. The nematic phase range is preferably at least 90xc2x0, in particular at least 100xc2x0. This range preferably extends at least from xe2x88x9220xc2x0 to +80xc2x0.
Measurements of the xe2x80x9ccapacity holding ratioxe2x80x9d (HR) [S. Matsumoto et al., Liquid Crystals 5, 1320 (1989); K. Niwa et al., Proc. SID Conference, San Francisco, June 1984, p. 304 (1984); G. Weber et al., Liquid Crystals 5, 1381 (1989)] have shown that mixtures according to the invention comprising compounds of the formula I exhibit a considerably smaller decrease in the HR with increasing temperature than do analogous mixtures in which the compounds of the formula I are replaced by cyanophenylcyclohexanes of the formula 
or esters of the formula 
The UV stability of the mixtures according to the invention is also considerably better, i.e. they exhibit a significantly smaller decrease in the HR on exposure to UV radiation.
The media according to the invention are preferably based on a plurality (preferably two or more) of compounds of the formula I.
The individual compounds of the formulae I to XVIII and their sub-formulae which can be used in the media according to the invention are either known or can be prepared analogously to the known compounds.
Preferred embodiments are indicated below:
Medium comprises one or more compounds of the formulae I1 to I13: 
X is preferably F, Cl, CN, OCN, NCS, SCN, CF3, CF2H, OCF3, OCF2H, OCFHCF3, OCFHCH2F, OCFHC2HF, OCF2CH3, OCF2CH2F, OCF2CHF2, OCF2CF2CF2H, OCF2CF2CH2F, OCFHCF2CF3, OCFHCF2CHF2, OCFHCFHCF3, OCH2CF2CF3, QCF2CF2CF3, OCF2CFHCHF2, OCF2CH2CHF2, OCFHCF2CHF2, OCFHCFHCHF2, OCFHCH2CF3, OCH2CFHCF3, OCH2CF2CHF2, QCF2CFHCH3, OCF2CH2CHF2, OCFHCF2CH3, OCFHCFHCHF2, OCFHCH2CF3, OCH2CF2CHF2, OCH2CFHCHF2, OCF2CH2CH3, OCFHCFHCH3, OCFHCH2CHF2, OCH2CF2CH3, OCH2CFHCHF2, OCH2CH2CHF2, OCHCH2CH3, OCH2CFHCH3, OCH2CH2CHF2, OCClFCF3, OCClFCClF2, OCClFCHF2, OCFHCCl2F, OCClFCHF2, OCClFCClF2, OCF2CHCl2, OCF2CHCl2, OCF2CCl2F, OCF2CClFH, OCF2CClF2, OCF2CF2CClF2, OCF2CF2CCl2F, OCClFCF2CF3, OCClFCF2CHF2, OCClFCF2CClF2, OCClFCFHCF3, OCClFCClFCF3, OCCl2CF2CF3, OCClHCF2CF3, OCClFCF2CF3, OCClFCClFCF3, OCF2CClFCHF2, OCF2CF2CCl2F, OCF2CCl2CHF2, OCF2CH2CClF2, OCClFCF2CFH2, OCFHCF2CCl2F, OCClFCFHCHF2, OCClFCClFCF2H, OCFHCFHCClF2, OCClFCH2CF3, OCFHCCl2CF3, OCCl2CFHCF3, OCH2CClFCF3, OCCl2CF2CF2H, OCH2CF2CClF2, OCF2CClFCH3, OCF2CFHCCl2H, OCF2CCl2CFH2, OCF2CH2CCl2F, OCClFCF2CH3, OCFHCF2CCl2H, OCClFCClFCHF2, OCFHCFHCCJ2F, OCClFCH2CF3OCFHCCl2CF3, OCCl2CF2CFH2, OCH2CF2CCl2F, OCCl2CFHCF2H, OCClHCClFCF2H, OCF2CClHCClH2, OCF2CH2CCl2H, OCClFCFHCH3, OCF2CClFCCl2H, OCClFCH2CFH2, OCFHCCl2CFH2, OCCl2CF2CH3, OCH2CF2CClH2, OCCl2CFHCFH2, OCH2CClFCFCl2, OCH2CH2CF2H, OCClHCClHCF2H, OCH2CCl2CF2H, OCClFCH2CH3, OCFHCH2CCl2H, OCClHCFHCClH2, OCH2CFHCCl2H, OCCl2CH2CF2H, OCH2CCl2CF2H, CHxe2x95x90CF2, OCHxe2x95x90CFF2, CFxe2x95x90CF2, OCFxe2x95x90CF2, CFxe2x95x90CHF, OCFxe2x95x90CHF, CHxe2x95x90CHF, OCHxe2x95x90CHF, CF2CH2CF3, CF2CHFCF3 in particular F, Cl, CN, CF3, CHF2, OCF3, OCHF2, OCFHCF3, OCFHCHF2, OCFHGHF2, OCF2CH3, OCF2CHF2, OCF2CHF2, OCF2CF2CHF2, OCF2CF2CHF2, OCFHCF2CF3, OCFHCF2CHF2, OCF2CF2CF3, OCF2CF2CClF2, OCClFCF2CF3 or CHxe2x95x90CHF2.
Medium additionally comprises one or more compounds selected from the group consisting of the general formulae II to VIII: 
in which the individual radicals have the following meanings:
R0: n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, in each case having up to 9 carbon atoms,
X0: F, Cl, halogenated alkyl, alkenyl or alkoxy having 1 to 6 carbon atoms,
Z0: xe2x80x94C2H4xe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94OCF2, xe2x80x94CF2Oxe2x80x94 or xe2x80x94C2F4xe2x80x94,
Y1, Y2, Y3 and Y4: in each case, independently of one another, H or F,
r: 0 or 1.
The compound of the formula IV is preferably 
The medium additionally comprises one or more compounds selected from the group consisting of the general formulae IX to XVIII: 
in which R0, X0, Y1 and Y2 are each, independently of one another, as defined above. X0 is preferably F, Cl, CF3, OCF3, OCHF2. R0 is preferably alkyl, oxaalkyl, fluoroalkyl or alkenyl, in each case having up to 6 carbon atoms.
The medium additionally comprises one or more compounds having fused rings of the formulae A-1 to A-6: 
xe2x80x83in which R0 is as defined above.
The proportion of compounds of the formula A-1 to A-6 is 0-20% by weight, preferably 3-15% by weight, in particular 3-10% by weight.
The proportion of compounds of the formulae I to VIII together is at least 30% by weight, preferably at least 50% by weight, in the total mixture;
The proportion of compounds of the formula I is from 1 to 50% by weight, preferably 2-30% by weight and in particular 5-25% by weight, in the total mixture;
The proportion of compounds of the formulae II to VIII is from 20 to 80% by weight in the total mixture 
The medium comprises one or more compounds of the formulae II, III, IV, V, VI, VII or VIII;
R0 is straight-chain alkyl or alkenyl having 2 to 7 carbon atoms;
The medium essentially consists of compounds of the formulae I to VIII;
The medium preferably comprises one, two or three compounds of the formula I;
The medium comprises a mixture of compounds of the formula I in which R1 is methyl, ethyl, n-C3H7, n-C4H9, n-C4H9, n-C5H1l or n-C6H11;
The medium comprises further compounds, preferably selected from the following group consisting of the general formulae XIX to XXII: 
xe2x80x83in which R0 and X0 are as defined above, and the 1,4-phenylene rings may be substituted by methyl,CN, chlorine or fluorine. The 1,4-phenylene rings are preferably monosubstituted or polysubstituted by fluorine atoms.
The medium preferably comprises carbocyclic dinuclear compounds of the formula XXIII 
xe2x80x83where 
xe2x80x83and 
xe2x80x83each independently of one another are 
Z0xe2x80x2 is a single bond, xe2x80x94C2H4xe2x80x94, xe2x80x94C4H8xe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94CF2Oxe2x80x94 or xe2x80x94OCF2xe2x80x94,
r0 is 1 or 2,
R0xe2x80x2 is as defined for R0 and
X0xe2x80x3 is OCF3, F, Cl, CF3, alkyl or alkoxy.
Preferred subformulae of the formula XXIII are 
xe2x80x83Very particularly preferred subformulae of the formula XXIII are 
xe2x80x83where
Alkyl is a straight-chain alkyl radical having 1-8 carbon atoms, especially having 2-5 carbon atoms.
The medium preferably comprises two or three compounds of the formulae XXIII.
The proportion of the compounds of the formula XXIII in the medium according to the invention is 5-40% by weight, especially 5-35% by weight.
The I: (II+III+IV+V+VI+VII+VIII) weight ratio is preferably from 1:10 to 10:1.
The medium essentially consists of compounds selected from the group consisting of the general formulae I to XVIII.
The term xe2x80x9calkylxe2x80x9d covers straight-chain and branched alkyl groups having 1-7 carbon atoms, in particular the straight-chain groups methyl, ethyl, propyl, butyl, pentyl, hexyl and heptyl. Groups having 2-5 carbon atoms are generally preferred.
The term xe2x80x9calkenylxe2x80x9d covers straight-chain and branched alkenyl groups having 2-7 carbon atoms, in particular the straight-chain groups. In particular, alkenyl groups are C2-C71E-alkenyl, C4-C7-3E-alkenyl, C5-C7-4-alkenyl, C6-C7-5-alkenyl and C7-6-alkenyl, in particular C2-C7-1E-alkenyl, C4-C7-3E-alkenyl and C5-C7-4-alkenyl. Examples of preferred alkenyl groups are vinyl, 1E-propenyl, 1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl, 3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl, 4E-hexenyl, 4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groups having up to 5 carbon atoms are generally preferred.
The term xe2x80x9cfluoroalkylxe2x80x9d preferably covers straight-chain groups containing terminal fluorine, i.e. fluoromethyl, 2-fluoroethyl, 3-fluoropropyl, 4-fluorobutyl, 5-fluoropentyl, 6-fluorohexyl and 7-fluoroheptyl. However, other positions of the fluorine are not excluded.
The term xe2x80x9coxaalkylxe2x80x9d preferably covers straight-chain radicals of the formula CnH2n+1xe2x80x94Oxe2x80x94(CH2)m, in which n and m are each, independently of one another, from 1 to 6. n is preferably 1 and m is preferably from 1 to 6.
It has been found that even a relatively small proportion of compounds of the formula I mixed with conventional liquid-crystal materials, but in particular with one or more compounds of the formula II, III, IV, V, VI, VII and/or VIII, results in a significant reduction in the threshold voltage and in low birefringence values, and at the same time broad nematic phases with low smectic-nematic transition temperatures are observed, thus improving the shelf life. Particular preference is given to mixtures which, in addition to one or more compounds of the formula I, comprise one or more compounds of the formula IV, in particular compounds of the formula IVa and/or IVd in which X0 is F, OCHF2 or OCF3. The compounds of the formulae I to VIII are colourless, stable and readily miscible with one another and with other liquid-crystal materials. Moreover, the mixtures of the invention are also notable for very high curing points, the values for the rotational viscosity xcex31 being comparatively low.
Through a suitable choice of the meanings of R0 and X0, the addressing times, the threshold voltage, the gradient of the transmission characteristic lines, etc., can be modified as desired. For example, 1E-alkenyl radicals, 3E-alkenyl radicals, 2E-alkenyloxy radicals and the like generally give shorter addressing times, improved nematic tendencies and a higher ratio between the elastic constants k33 (bend) and k11 (splay) compared with alkyl and alkoxy radicals. 4-Alkenyl radicals, 3-alkenyl radicals and the like generally give lower threshold voltages and lower values of k33/k11 compared with alkyl and alkoxy radicals.
A xe2x80x94CH2CH2xe2x80x94 group generally results in higher values of k33/k11 compared with a simple covalent bond. Higher values of k33/k11 facilitate, for example, flatter transmission characteristic lines in TN cells with a 90xc2x0 twist (for achieving grey tones) and steeper transmission characteristic lines in STN, SBE and OMI cells (greater multiplexibility), and vice versa.
The optimum mixing ratio of the compounds of the formulae I and II+III+IV+V+VI+VII+VIII depends substantially on the desired properties, on the choice of the components of the formulae I, II, III, IV, V, VI, VII and/or VIII and on the choice of any other components which may be present. Suitable mixing ratios within the abovementioned range can easily be determined from case to case.
The total amount of compounds of the formulae I to XVIII in the mixtures according to the invention is not crucial. The mixtures may therefore contain one or more further components in order to optimize various properties. However, the effect observed on the addressing times and the threshold voltage is generally greater the higher the total concentration of compounds of the formulae I to XVIII.
In a particularly preferred embodiment, the media according to the invention comprise compounds of the formulae II to VIII (preferably II, III and/or IV, in particular IVa) in which X0 is F, OCF3, OCHF2, OCHxe2x95x90CF2, OCFxe2x95x90CF2 or OCF2xe2x80x94CF2H. A favourable synergistic effect with the compounds of the formula I results in particularly advantageous properties. In particular, mixtures comprising compounds of the formula I and of the formula IVa are distinguished by their low threshold voltages.
The construction of the MLC display according to the invention from polarizers, electrode base plates and electrodes with surface treatment corresponds to the construction which is conventional for displays of this type. The term conventional construction here is broadly drawn and also covers all derivatives and modifications of the MLC display, in particular also matrix display elements based on poly-Si TFTs or MIMs.
An essential difference between the displays according to the invention and those customary hitherto based on the twisted nematic cell is, however, the choice of the liquid-crystal parameters in the liquid-crystal layer.
The liquid-crystal mixtures which can be used according to the invention are prepared in a manner which is conventional per se. In general, the desired amount of the components used in the lesser amount is dissolved in the components making up the principal constituent, expediently at elevated temperature. It is also possible to mix solutions of the components in an organic solvent, for example in acetone, chloroform or methanol, and, after thorough mixing, to remove the solvent again, for example by distillation.
The dielectrics may also contain other additives known to those skilled in the art and described in the literature. For example, 0-15% of pleochroic dyes or chiral dopes can be added.
C denotes a crystalline phase, S a smectic phase, SC a smectic C phase, N a nematic phase and I the isotropic phase.
V10 denotes the voltage for 10% transmission (view angle perpendicular to the plate surface) ton denotes the switch-on time and toff the switch-off time at an operating voltage corresponding to 2.5 times the value of V10. xcex94n denotes the optical anisotropy and no the refractive index. xcex94xcex5 denotes the dielectric anisotropy (xcex94xcex5=xcex581 xe2x88x92xcex5xe2x8axa5, where xcex5∥ is the dielectric constant parallel to the longitudinal molecular axes and xcex5xe2x8axa5 is the dielectric constant perpendicular thereto). The electro-optical data were measured in a TN cell at the 1st minimum (i.e. at a dxc3x97xcex94n value of 0.5) at 20xc2x0 C., unless expressly stated otherwise. The optical data were measured at 20xc2x0 C., unless expressly stated otherwise.
In the present application and in the examples below, the structures of the liquid-crystal compounds are indicated by acronyms, with the transformation into chemical formulae taking place in accordance with Tables A and B below. All radicals CnH2n+1 and CmH2m+1 are straight-chain alkyl radicals containing n or m carbon atoms, respectively; n and m are preferably 0, 1, 2, 3, 4, 5, 6 or 7. The coding in Table B is self-evident. In Table A, only the acronym for the base structure is given. In individual cases, the acronym for the base structure is followed, separated by a hyphen, by a code for the substituents R1, R2, L1 and L2:
Preferred mixture components of the mixture concept according to the invention are shown in Tables A and B.
Particularly preferred mixtures comprise, in addition to one or more compounds of the formula I, one, two, three, four, five or more compounds from Table B.
The entire disclosure of all applications, patents and publications, cited above and of corresponding application No. DE No. 19959797.9, filed December 11, 1999 is hereby incorporated by reference.